All three were on the plane.

It could have been arrogance, or it could have been budgetary. See, the Kremlin had been messing with Prigozhin's finances. So all the Kremlin had to do from there was set up a meeting in a city away from wherever Wagner was operating, and then insist that all three leaders be there.

They had to show up, and they only had the budget for one plane/security escort. And who would shoot down a plane? (Ignoring MH17)